Capacitor-grade lead wires with increased tensile strength and hardness

ABSTRACT

A capacitor-grade wire made from powder metallurgy containing at least niobium and silicon, wherein the niobium is the highest weight percent metal present in the niobium wire. The wire having a controlled tensile strength at finish diameter exceeds the strength of capacitor-grade wire formed by ingot metallurgy. Also, the powder metallurgy wire hardness exceeds capacitor-grade wire formed from ingot metallurgy with electrical leakage meeting the specifications normally applied to capacitor grade tantalum, niobium or niobium-zirconium lead wire at sinter temperatures of about 1150° C. and above.

This application is based on International Application No.PCT/US03/01822, filed Jan. 21, 2003, which was based on ProvisionalApplication No. 60/351,554 filed Jan. 24, 2002.

FIELD OF THE INVENTION

The invention relates generally to capacitor lead wires, moreparticularly to niobium lead wires usable with anode compacts oftantalum or niobium. The invention includes niobium powder metallurgyderived lead wires of niobium doped with silicon, preferably havingimproved strength and hardness without significant detriment toelectrical leakage rating of the wire.

BACKGROUND OF THE INVENTION

Niobium and niobium alloy lead wires with melt source derivation havebeen used as capacitor lead wires. Pure niobium wires of melt processorigin have low electrical leakage at sintering temperatures of 1150° C.and above. However the wires are limited in tensile strength andhardness, which make them difficult to work with; this results in lowproduction through put when bonding the wires to the capacitor anodecompacts and/or in the course of sintering the compact or prolysis ofsolid electrolyte with the lead wire attached. Niobium alloys, such asniobium-zirconium have better tensile strength then pure niobium wiresof melt process origin and acceptable electrical leakage above 1150° C.However above 1050° C. zirconium diffuses off the wire and contaminatesthe anode, making it unacceptable as a capacitor lead wire.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve chemical,mechanical, metallurgical, and functional consistency of capacitor gradelead wires.

It is a further object of the present invention to reduce sintering andbonding problems.

It is yet a further object of the present invention to improve niobiumwire to overcome the above-described disadvantages without significantlyimpacting the electrical properties of the wire and wire-anode assembly.

The invention relates to a process for making a capacitor gradesilicone-doped niobium lead wire comprising (a) forming a low oxygenniobium powder by hydriding a niobium ingot or a niobium bar andgrinding or milling the ingot or the bar, and thereby making a powderhaving a Fisher Average Particle Diameter particle size range of lessthan about 150 microns, (b) dehyriding the powder, and optionallydeoxidizing the powder, forming a low oxygen niobium powder, (c)blending the low oxygen niobium powder with a silicon additive powderand compacting the powder by cold isostatic pressing to a bar; (d)thermomechanically processing the bar into a rod, and (e) subjecting therod to a combination of rolling and cold drawning steps, and forming thesilicon doped wire. The invention also relates to a method made fromsuch a process.

The present invention includes a niobium wire made from powdermetallurgy (P/M), containing a silicon additive of less than about 600ppm. Generally, the amount of silicon ranges from about 150 to about 600ppm. Preferably, the amount of silicon ranges from about 150 to 300 ppm.The invention imparts a controlled, higher mechanical tensile strengthin the niobium wire at finish diameter that exceeds capacitor-grade wireformed from niobium and niobium-zirconium alloys derived directly fromingot metallurgy (I/M). Preferably too the P/M source niobium has oxygencontent below 400 ppm, even when silicon is added in an oxide form. TheP/M derived niobium, and niobium-silicon wires also have increasedhardness that exceeds hardness of capacitor-grade wire of I/M niobiumand niobium-zirconium wires and electrical leakage within currentspecifications at sinter temperatures of about 1150° C. and above, orabout 1250 and above. The P/M source material if sintered at well belowabout 1150° C. or 1250° C. and above, and/or attached to anode compactssintered below about 1150° C. or below 1250° C. would have higherleakage. But at about 1150° C. or 1250° C. and above, the differencesbecome minimal.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are intended to provide further explanation of the present inventionas described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart of the ultimate tensile strength as a function of wirediameter of select niobium and niobium alloy wire of the presentinvention derived from powder metallurgy compared to niobium and niobiumalloy wire derived from ingot metallurgy;

FIG. 2 is a chart of electrical DC leakage as a function of sinteringtemperature of select niobium and niobium alloy wire of the presentinvention derived from powder metallurgy compared to niobium and niobiumalloy wire derived from ingot metallurgy;

FIG. 3A-3F are side and front views of examples of capacitor lead wiresbonded to anode compacts; and

FIG. 4 is a chart of electrical DC leakage as a function of sinteringtemperature of select niobium and niobium alloy wire of the presentinvention derived from powder metallurgy compared to niobium and niobiumalloy wire derived from ingot metallurgy.

DETAILED DESCRIPTION OF THE INVENTION

One of the preferred embodiments of the invention is a lead wire ofsilicone-doped niobium made as follows. Niobium powders are formed byhydriding an ingot or bar of niobium and grinding or otherwise millingthe ingot or bar to create a powder at a size range of less than 150microns FAPD (Fisher Average Particle Diameter), dehyriding anddeoxidating. The hydride-grind process as disclosed in U.S. Pat. No.3,295,951 of Fincham et al and the deoxidation (with a combineddehydriding deoxidation) is described in U.S. Pat. No. 6,261,337 ofKumar, incorporated herein by reference in their entirety, both saidpatents are of common assignment with this application and Mr. Kumar asa joint inventor of the present invention. The niobium powder preferablyis attained with an oxygen level below 400 ppm, preferably below 200ppm. A silicon additive powder is blended with the low oxygen niobiumpowder, compacted by cold isostatic pressing (at up to 60 KSI) to apreform billet for extrusion or sinter bar preferably yeilding a barapproximately 1.3 inches diameter. The bar is thermomechanicallyprocessed to a rod. The rod is then rolled (or swaged) and cold drawn,typically with a schedule of reductions and intermediate anneals asfollows:

Annealed at 2500° F. for 1.5 hours;

Rolled to 0.440 inches diameter;

Annealed at 2500° F. for 1.5 hours;

Reduced to 0.103 inches diameter;

Drawn to 0.0346 inches diameter wire;

Drawn to a finish diameter.

Stated in general terms, the rod can be rolled (or swaged) and colddrawn, typically with a schedule of reductions and intermediate annealsas follows:

Annealed at a temperature ranging from about 2100° F. to about 2700° F.for a time ranging from about 0.5 hours to about 2.0 hours;

Rolled from a diameter ranging from about 1 inch to about 0.25 inchesdiameter;

Annealed at a temperature ranging from about 2100 to about 2700° F. fora time ranging from about 0.5 hours to about 2.0 hours;

Reduced from about 1 inch to about to 0.075 inches diameter;

Drawn to a finish diameter.

The diameter of the wire made in accordance to the invention can rangefrom about 0.005 inches to about 0.1 inches. The wire of the presentinvention can contain other additional ingredients such as other metalsor ingredients typically added to niobium metal, such as tantalum,zirconium, titanium, or mixtures thereof. The types and amounts of theseadditional ingredients can be the same as those used with conventionalniobium and would be known to those skilled in the art. TABLE 1 belowlists the chemistry of the specimens used in certain Experiments 1–5 ofsilicon doped niobium wire of powder metallurgy origin as reduced to 0.5inch diameter and 0.103 inch diameter.

TABLE 1 PPM C O N Mg Al Si Ti Cr Fe Ni Cu Zr Mo Ta W Experiment #1 ½″ 88646 47 114 20 25 20 108 655 157 10 10 20 1388 200 Experiment #2 ½″ 90301 42 106 20 158 20 99 574 133 16 10 20 8374 200 Experiment #3 ½″ 54322 60 120 0.5 13 6.1 45 225 44 4 5 1 3000 5 Experiment #4 ½″ 142 358 60120 1.1 161 5.3 50 255 53 3.5 5 1 10000 7.1 Experiment #5 ½″ 58 329 7295 2.7 306 5.5 45 230 53 7 5 1 20000 7.5 Experiment #1 .103″ 63 173 31110 2 23 2 140 500 130 4 5 11 1000 55 Experiment #2 .103″ 71 180 28 1053 163 2 150 675 150 6.4 5 11 10000 85 Experiment #3 .103″ 57 262 49 855.2 12 7.5 65 100 55 1.9 5 1 5000 6.8 Experiment #4 .103″ 79 291 52 1004.1 162 6.1 63 130 65 2.2 5 1 10000 5.7 Experiment #5 .103″ 61 282 59 802.8 294 4.9 63 70 55 1.9 5 1 10000 6.5

Wires were prepared from the silicon master blends presented inExperiments 1–5 of TABLE 1, and sample were taken at various sizemilestones and tested for tensile strength and hardness (Rockwellhardness B scale, HRB). I/M derived niobium-zirconium wires (prior art)were also tested similarly.

TABLE 2 Prior Art Nb PM Nb PM Nb PM Nb PM Nb PM NbZr Exp. #1 Exp. #2Exp. #3 Exp. #4 Exp. #5 Ingot (25 ppm) (150 ppm) (10 ppm) (150 ppm) (300ppm) Size Hardness Tensil Hardness Tensil Hardness Tensil HardnessTensil Hardness Tensil Hardness Tensil In HRB KSI HRB KSI HRB KSI HRBKSI HRB KSI HRB KSI 0.6 83.7 73 74.3 75.7 76.5 80.2 0.42 82.4 74.9 73.236.7 39.7 43.1 0.266 89.8 74.4 71 74.3 76.9 79.1 0.166 89.1 74.5 76.679.9 81 81.1 0.107 87.7 72 81 82 82.5 84.7 0.103 79.2 85.6 86.1 84.486.4 87.5 0.0933 68.5 41 80.8 53 76.9 55.6 0.0845 72.3 47 78.7 57.1 79.558.32 0.0765 71.6 47.2 81.4 59.72 82.7 62.5 0.0693 72.7 52.8 83.4 62.1282.4 64.86 0.0627 75.4 55 82.4 68.3 83.7 69.9 0.0568 75.4 55.9 85 72.5384.3 75.1 0.0514 76.9 62.5 83.7 75.6 85.4 77.7 89 119.88 91.5 122.28 98125.94 0.0465 77.2 64.4 84 76.1 86.3 78.7 87 124.65 90.5 130.17 96.8132.48 0.0422 78.3 66.7 85.4 81.28 84.7 82.7 92.5 126.05 91.7 133.4997.4 132.83 0.0382 79 65.5 86.5 83.5 85.8 84.2 88.3 131.23 93.2 138.4397.6 137.2 0.0344 85 70.31 88.5 89 85.6 87.7 90 130.57 92.5 143.76 97.5139.88 0.02878 83.7 71.22 86.5 93.8 87.1 94.6 93 133.74 94.2 142.57 99.6141.34 0.02634 84.7 72.21 88.5 95.2 88.5 96.3 96.7 150.2 99.7 154.8 99.7174.64 0.02431 85 72.93 89 101 89.5 99.7 96.4 168.63 98 180.61 98.1182.2 0.0223 87.3 74.63 89 99.3 89.9 103.3 99.3 178.14 99.4 180.66 100.3182.4 0.02062 87.6 75.88 90.5 103.4 91.4 106.8 98.8 188.97 100.2 206.8699.7 192.47 0.01995 87.8 83.56 90.7 112.32 90.7 114.98 99.7 164.45 100.2172.85 102 158.6 0.0173 85 82.30 90.1 116.8 90.5 117.66 100.5 168.54101.5 179.12 101.6 166.84 0.01537 86.8 73.36 91 119.56 91.2 121 99.7172.73 103.6 182.28 102.2 172.94 0.01334 87.8 73.36 90.6 126.95 91128.43 100 176.76 104.6 187.1 102.2 179.5

As can be seen from the results in TABLE 2 and FIG. 1, theniobium-silicon wire had a much higher tensile strength and hardnessthan the niobium-zirconium wire at about 0.050 inches diameter andbelow.

Also, electrical leakage tests (40 volts at 90%) were conducted for wire(wire-anode assemblies in capacitor test conditions) or anodes withselect silicon master blends (Experiments #1 and #2) and presented inFIG. 2. The tests were conducted for anode assemblies with lead wiresmade at various sintering temperatures. As can be seen from the resultsin TABLE 3 below and FIG. 2, the niobium-silicon wire is acceptable foruse at sintering temperatures of 1250° C. and above, but not lower,complying with the current tantalum capacitor grade wire specificationleakage of 0.6 μA/in² at 1250° C.

TABLE 3 (@1250° C.) Leakage μA/in² niobium ingot 0.1 niobium-zirconium0.25 Experiment #1 0.35 Experiment #2 0.6 Specification 0.6

Side and front views of examples of niobium-silicon capacitor lead wiresof the present invention bonded to anode compacts are illustrated inFIGS. 3A–3F. FIGS. 3A and 3B illustrate a niobium-silicon capacitor leadwire 10 butt welded to an anode compact 12. FIGS. 3C and 3D illustrate aniobium-silicon capacitor lead wire 10 imbedded for a length 14 withincompact 12. FIGS. 3E and 3F illustrated yet another attachment techniqueof welding the lead wire 10 to the top 16 of the compact 12. The leadwire 10 of any of FIGS. 3A–3F and/or the compact 12 of any such figurescan be circular or flat (ribbon form) or other shapes.

Also, electrical leakage tests (40 volts at 90%) were conducted for wire(wire-anode assemblies in capacitor test conditions) or anodes withselect silicon master blends (Experiments #3, 4 and 5) and presented inFIG. 4. The tests were conducted for anode assemblies with lead wiresmade at various sintering temperatures. As can be seen from the resultsin TABLE 4 below and FIG. 4, the niobium-silicon wire is acceptable foruse at sintering temperatures of 1150° C. and above, but not lower,complying with the current tantalum capacitor grade wire specificationleakage of 0.6 μA/in² at 1150° C.

TABLE 4 (@1150° C.) Leakage μA/in² niobium ingot 0.1 niobium-zirconium0.25 Experiment #3 0.09 Experiment #4 0.118 Experiment #5 0.103Specification 0.6

Artifacts of electrolyte impregnation and pyrolysis cathode attachmentand packaging all well known to those skilled in the art are omittedfrom the figures for convenience of illustration

Other embodiments of the present invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only.

1. A process for making a silicone-doped niobium lead wire comprising:(a) forming a low oxygen niobium powder by hydriding a niobium ingot ora niobium bar and grinding or milling the ingot or the bar, and therebymaking a powder having a Fisher Average Particle Diameter particle sizerange of less than 150 microns; (b) (i) dehydriding the niobium powder,and (ii) deoxidating the dehydrided niobium powder thereby forming a lowoxygen niobium powder; (c) blending the low oxygen niobium powder with asilicon additive powder, and compacting the powder blend of low oxygenniobium powder and silicon additive powder by cold isotactic pressing toform a bar; (d) thermomechanically processing the bar into a rod; and(e) subjecting the rod to the following sequential steps, (i) annealingat a temperature of about 2500° F. for 1.5 hours, (ii) rolling to adiameter of about 0.440 inches, (iii) annealing at a temperature ofabout 2500° F. for 1.5 hours (iv) reducing to a diameter of about 0.1inches, and (v) drawing to a wire having a diameter of at least about0.005 inches.
 2. The process of claim 1, wherein the silicon is added inan amount that is less than about 600 ppm.
 3. The process of claim 1,wherein the silicon is added in an amount ranging from about 150 toabout 300 ppm.
 4. The process of claim 1, wherein the wire furthercomprises a metal component selected from the group consisting oftantalum, zirconium, titanium, and combinations thereof.
 5. The processof claim 1, wherein the niobium powder has an oxygen level that is belowabout 400 ppm.
 6. The process of claim 1, wherein the wire has a tensilestrength exceeding capacitor-grade niobium wire and niobium-zirconiumalloys derived directly from ingot metallurgy.
 7. The process of claim 1wherein the niobium powder has an oxygen level of less than 200 ppm.